I thought you might be interested in this little work I've done.
Here is the list of jump locations of all predefined functions in debug yellow ROM (including glitch ones). Some of them jump to RAM, so may be they can be manipulated.
Also about predef 0x1E, which is stated to be nonfunctional in your list. Actually it calls GiveItem, which gives item based on registers BC: item ID comes from B, amount from C. I always got 32 Repels from using it (i checked in overworld and in battle), not sure, if it can be manipulated though.
P.S. Sorry for necrobumping